1. Field of the Invention
The present invention relates to a biaxially oriented polyester film which has excellent linearly tearable property at least in a longitudinal direction of the film, excellent strength, excellent heat resistance and excellent dimensional stability and is useful as a packaging material for foods (e.g. confectionery, pickle, soybean paste, soup, jam, etc.), frozen foods, refrigerated foods, retort-packed foods, drugs, daily necessities and cosmetics.
2. Description of the Related Art
Various packaging bags made of plastic films are widely used for packaging foods, drugs, miscellaneous goods, etc., and packaging bags produced by laminating two or more biaxially oriented plastic films and heat-sealable non-oriented plastic film are widely used.
The biaxially oriented polyester film is superior in durability, moisture resistance, mechanical strength, heat resistance and oil resistance, and biaxially oriented polyester films produced by a tubular method, a flat simultaneous biaxially orienting method, a flat successive biaxially orienting method and the like are used in the field of food packaging.
However, the packaging bags using these biaxially oriented polyester films have a problem that the tearing property is poor. In order to improve the tearing property, a method of providing a notch is used. However, there often arises a phenomenon that the packaging bag can not be linearly torn in case of tearing from this notch. In that case, not only the contents scatter on opening resulting in waste, but also clothes are sometimes soiled by an accident such as breakage of a soft confectionery such as cookie on opening, especially when the contents are liquid.
Examples of the easily openable material, which is superior in linearly tearable property when tearing the film, include a laminated film containing a monoaxial oriented polyolefin film as an intermediate layer. For example, there is a three-layer laminated film of a biaxially oriented polyester film/monoaxial oriented polyolefin film/non-oriented polyolefin film. In this case, it is necessary to provide an intermediate layer intentionally, thereby causing a problem of high cost. Therefore, use thereof was limited.
Therefore, as a method of imparting the linearly tearable property to the biaxially oriented polyester film itself, there has been suggested a biaxially oriented polyester film using as a main raw material a mixture comprising a thermoplastic polyester resin (i) wherein an acid component is mainly composed of terephthalic acid or naphthalenedicarboxylic acid, and a thermoplastic polyester resin (ii) capable of forming a phase-separation from the polyester resin (i) (see Japanese Patent Application KOKAI No. 169962/1996).
The above gazette discloses that polyethylene terephthalate (hereinafter referred to as "PET") is used as (i) and a polyether polyester block copolymer comprising polytetramethylene glycol (hereinafter referred to as "PTMG") and polytetramethylene terephthalate is used as (ii). However, when the production is conducted on a scale of mass production, for example, using an extruder (200 mm.phi.) or more), the unevenness of the film increased to cause problems such as difficulty in film formation, deterioration of the linearly tearable property of the resulting film, decrease in commercial value as the packaging bag due to an increase in haze of the film, etc.